Significant amounts of gold are found in sulfide ores, in which the gold is associated with sulfide mineralogy. The gold is difficult to recover from such sulfide ores, because the gold is typically bound in sulfide mineral grains in a manner that renders the ore refractory to many traditional gold recovery techniques, such as direct cyanidation of the ore. Therefore, sulfide ores are commonly treated to chemically alter the sulfide mineral to permit dissolution of the gold during subsequent gold recovery operations.
One technique for treating a gold-bearing sulfide ore in preparation for gold recovery is to subject the ore to an oxidative treatment to oxidize sulfide sulfur in the sulfide minerals, thereby rendering the gold more susceptible to recovery. One method for oxidatively treating a sulfide ore is pressure oxidation, in which a slurry of the ore is subjected to oxygen gas in an autoclave at elevated temperature and pressure to decompose the sulfide mineral, freeing the gold for subsequent recovery. Other oxidative treating methods include roasting and bio-oxidation of the ore in the presence of air or oxygen gas.
Treating whole ores by pressure oxidation or by oxidative roasting is expensive. Part of the expense is due to energy consumed in heating gold-barren gangue material in the whole ore, and especially the energy required to heat water in which the gangue material is slurried in the case of pressure oxidation. Also, process equipment for treating a whole ore must be sized to accommodate the throughout of gangue material, in addition to the throughput of the gold-bearing sulfide minerals, thereby significantly adding to the cost of process equipment. Moreover, side reactions may occur involving gangue material which can detrimentally affect the oxidative treating or can produce hazardous materials which require special handling.
One way to reduce the high energy and process equipment costs associated with oxidative treating of a whole ore, as well as the potential for problems associated with side reactions, would be to remove gangue material from the ore prior to the oxidative treatment. For example, one method that has been used to remove gangue material from gold-bearing sulfide ores is flotation. In flotation, air is bubbled through a slurry of ore particles which have been treated with reagents and the particles of the ore which are less hydrophilic tend to rise with the air bubbles, thereby permitting separation of the ore into two fractions. Flotation has been used to prepare concentrates of gold-bearing sulfide minerals which are rich in the sulfide minerals and relatively free of gangue material. One problem with flotation of many gold-bearing sulfide ores, however, is that a significant amount of the gold-bearing sulfide mineral often reports to the wrong flotation fraction, representing a significant loss of gold.
There is a significant need for an improved method for processing many gold-bearing sulfide ores that avoids the high costs associated with oxidatively treating whole ores without the significant loss of gold associated with concentrating sulfide ores by flotation.